1. Field of the Invention
The present invention relates to methods and systems for generating a magnetic resonance data file from raw magnetic resonance data acquired from an examination subject.
2. Description of the Prior Art
Magnetic resonance imaging is a widely used image modality, wherein an examination subject is moved into a strong, static basic magnetic field to cause nuclear spins in the examination subject, that were previously randomly oriented, to become aligned with the direction of the basic magnetic field. Radio-frequency (RF) energy is then radiated into the examination subject, causing the nuclear spins to be deflected from their aligned orientation. As the nuclear spins precess upon returning to the aligned orientation, they emit RF magnetic resonance signals that are detected and from which an image of an interior region of the examination subject can be constructed according to any number of known image reconstruction techniques.
For the purpose of spatially encoding the magnetic resonance signals, the examination subject is also in the presence of gradient fields, respectively generated by gradient coils, the gradients field typically being oriented along the respective axes of a Cartesian coordinate system, with the z-axis of this coordinate system corresponding to the longitudinal axis of the examination subject.
The received magnetic resonance signals are referred to as raw data, and the raw data are stored in a computer memory that represents a mathematical domain, also referred to as the spatial domain, known as k-space. The raw data are entered in k-space at respective points that are (usually) equidistantly spaced from each other, so as to form a grid-like format. For reconstructing an image from the raw data, the raw data are subjected to a Fourier transformation to transform the raw data into image data in the image domain, from which the image of the examination subject is reconstructed.
A certain amount of time is necessary in order to acquire a sufficient amount of magnetic resonance data in order to generate an image that is substantially free of artifacts and noise, and that has a sufficient contrast so that the diagnostic content of the image can be easily discerned. When obtaining a magnetic resonance image of an organ in the form of a static “snapshot,” although it is usually desirable to shorten the acquisition time for patient comfort and for the purpose of making use of the imaging apparatus in an efficient manner, a somewhat longer data acquisition time can be tolerated, if necessary in order to produce the aforementioned desirable characteristics of the resulting image.
A different situation exists, however, in so-called dynamic studies, wherein a physiological process that changes with respect to time is being imaged by magnetic resonance techniques. An example is magnetic resonance angiography (MRA), wherein typically a contrast agent is injected into the vascular system of a patient, and data acquisition of the region of interest must be timed to occur within a time window within which the contrast agent bolus is flowing through the region of interest. It is also desirable under many circumstances to obtain a relatively rapid series of magnetic resonance exposures as the contrast agent proceeds through the region of interest. Therefore, particularly in this context, it is desirable to be able to increase the time resolution of the generated images, i.e., to reduce the amount of time between the respective beginnings of data acquisition for each image.
Various techniques for improving the time resolution in magnetic resonance imaging are known that are based on the fact that the central region of k-space contains the most relevant data, or at least the data that are primarily the basis for obtaining an image with a good contrast. Various techniques are therefore known in the field of magnetic resonance imaging wherein the data in this central region of k-space are more frequently updated than the data in the peripheral regions of k-space. This type of k-space sampling is known as the keyhole technique, and is described, for example, in U.S. Pat. No. 5,754,046 and in “Composite k-Space Windows (Keyhole Techniques) To Improve Temporal Resolution in a Dynamic Series of Images Following Contrast Administration,” Brummer et al, SMRM, August 1992, Page 4236.
It is also known to increase the frame rate of a series of reconstructed magnetic resonance images in a dynamic MRA study by sampling the central region of k-space at a higher rate than the peripheral regions of k-space, as described in U.S. Pat. No. 5,713,358. In this technique, image frames are reconstructed at each sampling of the central k-space region, using the temporally nearest samples from the peripheral k-space regions.